Modern integrated circuits are made up of literally millions of active and/or passive devices such as transistors, capacitors, inductors, etc. These devices are initially isolated from each other, but are later interconnected together to form functional circuits. Typical interconnect structures include lateral interconnections, such as metal lines (wirings), and vertical interconnections, such as vias and contacts. Interconnections are increasingly determining the limits of performance and the density of modern integrated circuits. On top of the interconnect structures, bond pads are formed and exposed on the surface of the respective chip. Electrical connections are made through bond pads to connect the chip to a package substrate or another die. Bond pads can be used for wire bonding or flip-chip bonding.
Flip-chip packaging utilizes bumps to establish electrical contact between a chip's input/output (I/O) pads and the substrate or lead frame of the package. Structurally, a bump may contain the bump itself and a so-called under bump metallurgy (UBM) located between the bump and an I/O pad. The UBM may contain an adhesion layer, a barrier layer and/or a wetting layer. The bumps themselves, based on the material used, may be classified as solder bumps, gold bumps, copper pillar (or post) bumps, bumps with mixed metals, etc. However, there are challenges related to the formation and utilization of copper bump structures.